Add support for matchPattern:

This commit adds a new matcher, KWRegularExpressionPatternMatcher, that supports use of regular expression patterns in expectations. For example:

    [[@"ababab" should] matchPattern:@"(ab)+"];
    [[@"ababab" shouldNot] matchPattern:@"(abc)+"];
    
    [[@" foo " should] matchPattern:@"foo"];
    [[@" foo " shouldNot] matchPattern:@"^foo$"];

See https://gist.github.com/kristopherjohnson/5362002 for more examples.